This invention relates to audio delay systems.
Such systems are used in the music industry to produce various sound effects. An example of such a sound effect is the so-called "flange", in which an undelayed audio signal is mixed with a delayed audio signal, and the amount of delay is varied over time. When the two signals are mixed, they constructively interfere with each other at some frequencies to create reinforcements, while destructively interfering with each other at other frequencies to create cancellations. As the delay varies, the reinforcement and cancellation frequencies vary, producing the flange effect.
Typically, the delay is varied by varying the frequency of the clock controlling the rate at which the incoming analog audio signal is digitally encoded, and the rate at which the digitally encoded signal is decoded. For example, to obtain a four-to-one flange, the clock is varied over a four-to-one range. The four-to-one variation in clock frequency varies the time during which the encoded signal is resident in memory, and thus the delay, by the same four-to-one ratio. Because clock frequency variations can typically not be as wide as desired delay variations (because sound fidelity will deteriorate as the result of increased quantization noise), it has typically been necessary to switch into use a variable number of sequentially-arranged memory elements, each of which delays the encoded signal for a predetermined period of time. Switching memory elements into use in this manner creates audible encoding artifacts during the interval required to fill new memory elements with encoded data.